Between two wireless beacons, you could send a unicast packet, and receive an acknowledgement that follows every unicast packet. For multicast packets, there is typically no acknowledgement. In wired networks, this may not be a serious concern. However, if a wireless packet gets lost on air due to, by way of example but not limitation, collisions or interference, data is lost. Moreover, it would not be known whether retrying transmissions of multicast packets would be beneficial since there is no acknowledgement. So, multicast transmission typically does not include retrying transmission of multicast packets.
Although multicast traffic is efficient—since multicast packets require no acknowledgement from recipients, and no retry—wireless multicast data can be unreliable. This renders problematic the execution of certain applications that need relatively reliable data, such as, by way of example but not limitation, multicast video and multicast audio. Indeed, in dense deployments, multicast video applications are practically unusable.
Some ways to improve wireless multicast reliability require that multicast transmissions are essentially—or actually—turned into multiple unicast transmissions. For example, a system could require acknowledgement from all multicast packet recipients. As another example, a multicast packet could be converted into multiple unicast packets. These remedies may improve reliability, but would likely increase overhead and decrease efficiency. Multicast to unicast conversion is known to be costly. Acknowledgement solutions achieved with token passing or randomization are complex and, particularly in the case of randomization, unpredictable.
Another possible way to improve wireless multicast reliability is with media reservation schemes. Media reservation may be across full transmission time, i.e., Clear to Send (CTS) to self reserving for the full transmission time, but this does not solve for loss while packets are transmitted. Media reservation for every packet, i.e., CTS to self for every packet, is inefficient. Application layer solutions, on the other hand, have been implemented but solve the problem on the wrong layer. It would be desirable to provide wireless multicast functionality that is more efficient than that of these prior art systems, and more reliable than that of a system in which no acknowledgements are required.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.